In amplifiers it is often useful to have a warning before the amplifier's output reaches saturation to avoid significant distortion. Typically, the distortion grows gradually at first as the output signal level approaches saturation, and increases abruptly as the amplifier reaches saturation and is driven beyond the saturation onset. The saturation detection and warning is especially useful for audio amplifiers.
The actual output level where saturation occurs depends on a number of variables: process parameters, environmental variables like temperature, and application variables like load current, supply voltage, and others. In order to maximize the utilized output range of the amplifier it is advantageous to choose the threshold of the saturation detector and warning circuit as close to the output saturation level as possible, yet low enough to avoid significant distortion that results from output clipping (which occurs when the amplifier is driven into saturation).
Another application for a saturation detector and warning circuit is in Low Drop out Linear regulators (LDO-s) as a drop out detector and warning circuit. In these voltage regulators it is useful for the system that utilizes them (e.g. a portable system like cellular phone or PDA) to sense the impending loss of output voltage regulation and take appropriate action, like saving all data in before it happens. Hence, a circuit that detects the impending drop out of an LDO is useful.
For clarity of discussion it is worthwhile to point out that “saturation” is used in different ways when it refers to an amplifier or when it refers to a MOSFET transistor. In an amplifier, saturation means that the output of the amplifier hits its limit and essentially does not respond to further increase of its input voltage (the amplifier's output is essentially clamped, typically at close to the supply voltage). In other words the amplifier does not operate as a linear amplifier anymore in saturation. In an amplifier that utilizes feedback, i.e. most of the linear amplifiers, this means that the feedback loop is no longer closed. In contrast, if the amplifier is realized using MOSFET transistors the output MOSFET transistor typically operates in its non-saturated triode (or resistive) region when the amplifier is in, or close to, saturation. Consequently typically a detector circuit that detects the saturation or impending saturation of an amplifier actually detects that its output transistor(s) is operating in, or entering into, its non-saturated resistive (or triode) operating region.
FIG. 1 shows a prior art saturation detector and warning circuit. The amplifier 101 consists of an Amplifier Front End (AF) that drives the two output MOSFET transistors PMOS1 and NMOS1, forming an amplifier with rail-to-rail output voltage capability. Circuit 100 forms the saturation detector and warning circuit. The comparator SATCompH senses the onset of high side saturation of the amplifier by comparing the drain to source voltage of the high side output MOSFET, PMOS100, with a reference voltage SATREFH. SATREFH is typically a small voltage compared to the supply voltage of the amplifier Vcc. This voltage can be constant, e.g. 200 mV, in some embodiments, or can be a small fraction of the supply voltage Vcc, e.g. 0.05*Vcc, in other embodiments. Similarly, SATCompL and SATREFL sense the onset of low side saturation of the amplifier by comparing the drain to source voltage of the low side output MOSFET, NMOS100, with a reference voltage SATFREFL.
The logic “H” level at the output of comparator SATCompH indicates that the amplifier is close to high side saturation. Similarly logic “H” level at the output of comparator SATCompL indicates that the amplifier is close to low side saturation. The outputs of the two comparators are “OR” connected. The output of the “OR” gate is logic “H” if the amplifier is close to either low side or high side saturation, hence it can be used as a saturation detection or warning signal.
The disadvantage of the prior art solution is that it does not track the variation of the saturation voltage level caused by process, temperature supply voltage and load current variations. This results in over-design, as the threshold levels SATREFH and SATREFL have to be selected for worst case conditions. As a consequence, the prior art circuits may signal the onset of saturation too early under typical conditions, thereby limiting the usable output voltage swing of the amplifier.
Although some device numbers are duplicated across figures, i.e. MOS12 in FIG. 3, FIG. 4 and FIG. 5, the sizes and aspects of these elements need not be identical across embodiments.